A polymer of ethylene, and a copolymer comprising ethylene as a main portion and .alpha.-olefin (Hereafter, both compounds are called as an ethylene base polymer in this specification.) have been widely used as resin materials for various plastic products. Ethylene base polymers differ in required characteristics according to forming methods and applications each other. For example, polymers with a relatively low molecular weight and a narrow molecular weight distribution are suitable for products formed by injection molding and blow molding, whereas polymers with a relative high molecular weight and a wide molecular weight distribution are suitable for film forming and blow molding.
Molecular weight distribution relates to the surface texture of film or blown products. It has been considered that the wider the molecular weight distribution is, the smaller the roughness of the surface texture of the formed products is. Namely, even with polymers with a high molecular weight, products of high quality with good fluidity and no melt fracture on the surface of the formed products can be obtained. In addition, when a comparison is made among polymers with same molecular weight, one with a wider molecular weight distribution brings about ethylene base polymers with high density and, as a result, improves the mechanical strength of products. Moreover, when a comparison is made among polymers with same density, one with a wider molecular weight distribution improves the environmental stress cracking resistance (ESCR) of products. On the other hand, when a comparison is made among polymers with same molecular weight, one with a wider molecular weight distribution increases the density and crystallization and, as a result, decreases shock resistance. Accordingly, an ethylene base polymer with a wide molecular weight distribution which is excellent in shock resistance, and environmental stress cracking resistance (ESCR), and causes no melt fracture for film forming and blow molding have long been waited for.
It has been known fact that an ethylene base polymer with a wide molecular weight distribution can be obtained using a catalyst, generally known as a Phillips catalyst, which is made by carrying chromium trioxide or any compounds containing chromium able to be oxidized and changed into chromium trioxide under a certain activation condition carried on a solid inorganic oxide such as silica. However, an ethylene base polymer which has been prepared by a Phillips catalyst has a molecular weight distribution (Mw/Mn) of about 8 to 30 which is expressed as the ratio of weighted average molecular weight (Mw) to numbered average molecular weight (Mn). But, in some application, an ethylene base polymer with a wider molecular weight distribution than the above polymer is required. And also there is a problem in that ESCR is extremely low. Japanese Patent No. 2996/1969 (U.S. Pat. No. 3,324,095), Japanese Patent No. 3827/1969 (U.K. Patent No. 1,051,391), and Japanese Patent No. 1766/1972 (U.K. Patent No. 1,332,510) disclose that an ethylene base polymer with a wide molecular weight distribution can also be obtained using a catalyst where an ester of a chromic acid was carried on a solid inorganic oxide such as silica. An ethylene base polymer prepared using the above catalyst has fairly a wide molecular weight distribution with about 20 to 60 Mw/Mn and higher ESCR than that obtained using a Phillips catalyst, but, as a defect, has low shock resistance and causes melt fracture when at film forming or blow molding Therefore, this polymer has a problem in a practical application.
U.S. Pat. Nos. 5,104,841 and 5,137,997 disclose that an ethylene base polymer with a wide molecular weight distribution can also be obtained by using a catalyst where bis(trimethylsilyl)amide complex derived from a divalent chromium atom is carried on a solid inorganic oxide. Mw/Mn is as wide as 50 to 80. Though a molecular weight distribution is fairly wide, this catalyst is extremely low in polymerization activity so that there is a practical problem in productivity. Moreover, since bis (trimethylsilyl)amide complex derived from a divalent chromium atom is extremely sensitive to air, water, etc., it is very hard to handle so that this polymer has also a practical problem. In addition, this polymer has also another problem that the product has a low shock resistance.
Moreover, Japanese Patent Application Laid-open No. 503739/1995 (International Publication Number WO93/9149) discloses that an ethylene base polymer with a wide molecular weight distribution can be obtained by changing the molar ratio of alumoxane to chromium at polymerization using either of a catalyst where chromium acetylacetonate was carried on silica or a catalyst system using alumoxane. But, the preparation method is very complicated in controlling a process so that the method has a practical problem.